1. Field of the Invention
The present invention relates to a recording method and a recording apparatus utilizing the so-called thermal transfer process in which a recording material held in a transfer section is heated by heating means so as to fly toward a target member, onto which the recording material is to be transferred and which is placed opposite to the transfer section, thereby forming a predetermined transfer image on the target member.
2. Description of the Related Art
Recently, printing out such color images as processed on personal computers or picked up by video cameras and electronic still cameras, for example, has become increasingly popular for enjoyment and other purposes. That trend has increased a need for printers with the capability of producing high-quality full color images. In particular, high-quality full color images have begun to be required even in relatively inexpensive printers adapted for, e.g., personal users and small-scaled offices called SOHO (Small Offices and Home Offices).
As color printing processes, there are proposed so far color hard copy processes such as a sublimation thermal transfer process (or a dye diffusion thermal transfer process), a fusion thermal transfer process, an ink jet process, an electrophotographic process, and thermal-development silver salt process. Among these processes, the dye diffusion thermal transfer process and the ink jet process are particularly known as being able to readily produce a high-quality image with a relatively simple apparatus.
In the dye diffusion thermal transfer process, an ink layer containing a high-density transfer dye dispersed in an appropriate binder resin is coated on an ink ribbon or sheet, and the ink ribbon or sheet is brought into close contact with the so-called thermal transfer paper on which a dyeing resin accepting the transferred dye is coated. Heat is applied by a heat-sensitive recording head (thermal head) to the back side of the ink ribbon or sheet so that the transfer dye is thermally transferred from the ink ribbon or sheet onto the thermal transfer paper in accordance with the amount of heat applied.
By repeating the above operation for each of image signals decomposed corresponding to, for example, three primary colors in the subtractive color process, i.e., yellow (Y), magenta (M) and cyan (C), a full color image can be obtained which has a continuous gradation.
FIG. 71 shows the construction of a thermal head and thereabout of a printer in accordance with the dye diffusion thermal transfer process.
A thermal head 90 is disposed in an opposed relation to a platen roller 97. Between the thermal head 90 and the platen roller 97, an ink sheet 91 having an ink layer 93 coated on a base film 92 and a sheet of recording paper (thermal transfer paper) 94 having a dye resin layer (dye accepting layer) 96 coated on the surface of a sheet of paper 95, for example, run in the direction of arrow A in FIG. 71 while both the sheets are pressed against the thermal head 90 by the platen roller 97 rotating in the direction of arrow B.
Ink contained in the ink layer 93 is selectively heated by the thermal head 90 in accordance with an image to be printed, whereupon the ink is thermally diffused into the dye resin layer 96 of the recording paper 94 that is held in contact with the ink layer 93 and is hence under heating. As a result, a transfer image is formed in a dot pattern, for example.
The dye diffusion thermal transfer process is a superior technique with capabilities of reducing the size of a printer, making maintenance of the printer easier, providing immediate image formation, and producing an image with high quality comparable to that of silver salt color prints. The dye diffusion thermal transfer process however has major disadvantages in that because ink ribbons or sheets are discarded after once used, a large amount of wastes is generated and the running cost is increased. Another problems is that thermal transfer paper must be used as recording paper, which also pushes up the cost. Further, a transfer time as long as about one minute is required to form an A6-size image.
The fusion thermal transfer process enables an image to be transferred onto ordinary paper, but also has similar problems of generating a large amount of wastes and increasing the running cost because ink ribbons or sheets are discarded after once used,. Another problem is that image quality is inferior to that of silver salt prints.
The thermal-development silver salt process can produce a high-quality image, but also has similar problems of generating a large amount of wastes and increasing the running cost because specific printing paper and throwaway ribbons or sheets are used. Another problem is that the cost of an apparatus used for this process is high.
On the other hand, as the ink jet process, there are known an electrostatic attraction type, a continuous vibration generating type (piezoelectric type), a thermal type (bubble-jet type), etc. as disclosed in, for example, Japanese Patent Publication No. 61-59911 and No. 5-217. In any type, printing is performed such that small droplets of ink are forced to eject from nozzles provided in a printer head and stick onto a sheet of printing paper or the like.
With the ink jet process, therefore, an image can be transferred onto ordinary paper, and no use of ink ribbons, etc. makes it possible to hold down the running cost and essentially avoid generation of wastes. In response to needs for simple printing of color images, this process has become increasingly used recently.
However, the ink jet process (particularly the on-demand type ink jet process) has a difficulty in achieving a density gradation per pixel from its own principles, and hence has a difficulty in reproducing such a high-quality image in a short time as being obtainable with the dye diffusion thermal transfer process and comparable to that of silver salt prints. Stated otherwise, in the ink jet process, since one droplet of ink constitutes one pixel, it is difficult to achieve a density gradation per pixel from the standpoint of principles. For this reason, a high-quality image cannot be formed. Although a method of realizing a pseudo-gradation with the dither process by making use of a high resolution of the ink jet process is attempted, this method cannot provide image quality comparable to that obtainable with the dye diffusion thermal transfer process, and lowers the transfer speed considerably.
Lately, there have also appeared, for example, an ink jet process wherein diluted ink is employed to provide a two- or three-level gradation, and an ink jet process wherein the size of ink droplets is reduced. Using the diluted ink however has problems that transfer heads must be prepared in several levels, thus resulting in a higher head cost, and a large amount of solvents absorbed in the same pixel raises a difficulty in design of printing paper, thus resulting in a higher running cost. Further, the principles of the on-demand type ink jet process have such a limitation that it is difficult to reduce the size of each ink droplet down to be smaller than one picoliter. For this reason, a satisfactory fine density gradation of 64 or more levels cannot be obtained in a unit pixel. For an image such as a natural picture, particularly, the on-demand type ink jet process has difficulties in providing image quality comparable to that obtainable with the dye diffusion thermal transfer process and silver salt prints.
The electrophotographic process is realized with a low running cost and a high transfer speed, but cannot provide image quality comparable to that obtainable with silver salt prints, and in addition requires an apparatus having a considerably expensive cost.
As a color printing process that can satisfy the demands mentioned above, the so-called dye vaporization thermal transfer process has been proposed (see, e.g, Japanese Unexamined Patent Publication No. 9-183239 and No. 9-183246).
In the dye vaporization thermal transfer process, ink is heated in a transfer section of a printer head so as to fly based on such a phenomenon as vaporization or ablation. The flying ink sticks to the surface of a target member (a sheet of printing paper such as a printer sheet), onto which an image is to be transferred and which is placed opposite to the printer head with a gap on the order of, e.g., 50 to 100 .mu.m left between them, thereby forming a transfer image.
The transfer section has an ink holding structure in the form of such a concave/convex structure that a large number of pillar or columnar members having a width or diameter of about 2 .mu.m and a height of about 6 .mu.m, for example, are arranged to stand vertically with small gaps of about 2 .mu.m therebetween. A heater is provided in a lower portion of the ink holding structure to constitute a vaporizing section (transfer section).
With the transfer section including the above ink holding structure, the following advantages (1) to (4) are obtained.
(1) Ink is spontaneously supplied to the vaporizing section based on a capillary phenomenon. PA1 (2) Ink can be efficiently heated due to a large surface area. PA1 (3) By properly setting the height of the pillar members, a predetermined amount of ink can be always held in the vaporizing section. PA1 (4) Since the surface tension of a liquid generally exhibits a negative temperature coefficient, locally heated ink is subject to a force tending to move the ink toward an outer periphery at a lower temperature. The movement of the ink is however minimized by the ink holding structure, and hence a reduction in transfer sensitivity is prevented.
Accordingly, it is possible to fly ink in an amount corresponding to the amount of heat applied to the vaporizing section for transfer onto a sheet of printing paper or the like, to achieve continuous control of the amount of transferred ink, and to provide a density gradation in each pixel. As a result, a high-quality image comparable to that obtainable with silver salt prints, for example, can be obtained.
Also, no need of using ink ribbons and so on reduces the running cost. In addition, by using ink that has high absorption to ordinary paper, image transfer onto ordinary paper can be achieved and the use of ordinary paper also contributes to further reducing the running cost.
Moreover, since the dye vaporization thermal transfer process utilizes vaporization of ink, i.e., dyes, the transfer section of the printer head for heating the ink needs to be neither pressed against a transfer target member such as a sheet of printing paper under a high pressure, nor brought into contact therewith. This obviates a problem of thermal fusion adhesion between an ink heating member such as an ink ribbon and a transfer target member such as a sheet of printing paper, which problem possibly occurs in the other thermal transfer processes.
With the conventional dye vaporization thermal transfer process disclosed in Japanese Unexamined Patent Publication No. 9-183239 and No. 9-183246, however, since ink is forced to fly for transfer with vaporization or ablation under heating and most of the flying ink is a vaporized matter in the form of a single molecule (including a small mist particle with a diameter of 1 .mu.m or less that is created by condensation of ink molecules in the gap), the transfer sensitivity (OD=optical density) is reduced (that is to say, the transfer speed is lowered) and the reproducibility may differ depending on quality of printing paper. Further, since the volume of the flying ink is too small, the flying ink loses a speed at once, and the gap between the transfer section of the printer head and the transfer target member cannot be increased in length. This results in that paper powder or dust adhering to the surface of the transfer target member may in turn adhere to the transfer section and cause unevenness in a transfer image.
Also, while ink is vaporized or ablated in the transfer section, impurities such as silica particles and metal powder, which are contained in trace amount in the ink and have the relatively high boiling points, are hard to vaporize and are accumulated in the transfer section. Eventually, fusing of the accumulated impurities may occur and deteriorate a transfer capability over time. In addition, if the boiling point of a solvent in the ink is much lower than that of dyes therein, only the solvent is selectively evaporated, causing the dyes to precipitate in the transfer section. This results in a problem that a range in which used solvents are selectable is narrowed.